Method of forming a cellulose sponge



United States Patent 3,261,704 METHOD OF FORMING A CELLULOSE SPONGE Fred B. Stieg, 35-16 85th St, Jackson Heights, N.Y. No Drawing. Filed Apr. 3, 1959, Ser. No. 803,848 7 Claims. (Cl. 106-122) This application is a continuation-in-part of my copending applications, Serial No. 402,107, filed January 4, 1954, now US. Patent No. 2,880,726 and Serial No. 739,420, filed June 2, 1958.

This invention relates to synthetic sponges, particularly to a method of forming sponges of regenerated cellulosic material.

While various forms of regenerated cellulosic sponges have been proposed and certain of these have been commercially used, these known types of regenerated cellulose sponges have been found to be feasible only for a limited number of uses, both from a purely physical viewpoint and also from an economic standpoint. In some instances, the known types of cellulose sponges have not been well adapted to articles or uses wherein aligned fibres, for example, surgical cotton, have been widely used. Another difficulty experienced with presently known synthetic sponges has been the inability to proper- 1y balance or proportion their dimensioanl stability, liquid absorbency, desired firmness when dry with desired softness when Wet, mechanical strength, rapidity of liquid absorbency and liquid retention under slight or moderate compression.

It is an object of the present invention to overcome the foregoing disadvantages and difficulties of known cellulosic sponges and to provide a method of forming sponges which will have characteristics suitable for a wide variety of uses. Other objects and advantages of the invention will be in part apparent and in part pointed out hereinafter.

In one of its broader aspects, the invention may be said to lie in providing a method of forming a regenerated cellulose sponge of extremely high porosity. In another aspect the invention may be said to lie in providing a method of forming a sponge of such high porosity, which has good mechanical strength without the addition thereto of fibres to reinforce the regenerated cellulose comprising the sponge. In still another aspect the invention may be said to lie in providing a method of forming regenerated cellulose sponges which contain significant amounts of powdered or comminuted cellulosic material, preferably highly purified cellulose which is commonly called alpha cellulose. In still another aspect, the invention may be said to lie in providing a method of forming a sponge comprised of highly purified, bleached and sterile cellulose.

Before describing my invention in detail, it is to be understood that reference to specific embodiments of my invention is primarily intended to aid in illustrating the features and advantages of the invention but is not intended to limit the invention, which is defined with more formal particularity in the appended claims.

The following example is one of the presently preferred forms of my invention; sheets of high viscosity purified (alpha grade) cellulose are first treated with 18% to 20% caustic soda solution to yield alkali cellulose; and the swollen sheets of this alkali cellulose in turn are compacted or squeezed back to approximately the original sheet thickness to rid the sheets of caustic soda solution to the extent of that portion desired to be retained. Then the sheets are disintegrated or shredded to form alkali cellulose crumb. The alkali cellulose crumb is next treated with carbon disulphide to form cellulose Xanthate. To this is added dilute NaOH solution to yield a viscose solution, which, when ready for use, is diluted with water or with dilute NaOH solution to arrive at a. viscose solution containing from 4% to 6% cellulose in the form of xanthate and about 4% to 6% NaOH. To this viscose mixture, fine crystals of a hydrate of trisodium phosphate (which pass a 30 mesh screen or finer) are added to form a paste with a consistency suitable for molding or extruding. The paste is then placed in a one inch diameter mold and subjected to an initial elevated temperature between 70 C. to about C. for 30 minutes. After this initial heating the mass has assumed a homogeneous gel-like condition and further heat is then applied. This further heat which is above 75 C., is applied until the regeneration is complete and the crystals are fully dissolved. As a final step the formed sponge is Washed free of excess chemicals.

In one procedure, where powdered or comminuted cellulose material, as such, is a part of the articles to be formed, the comminuted cellulose is dispersed in the water to be added to arrive at the 4% to 6% cellulose, about 4% to 6% NaOH, viscose solution.

In another procedure, wherein meroerized powdered or comminuted cellulose is to be a part of the final articles, the powdered cellulose is separately mercerized with about a 940% caustic solution; enough solution is used so that when it is later diluted with Water and added to the viscose solution, the cellulose concentration (in the form of xanthate) of the final mixture is from 4% to 6% and the aOH about 4% to 6%.

To the viscose mixtures described above ll add powder fine crystals of a hydrate of an alkali metal salt as the pore forming material to form a paste with a consistency suitable for molding or extruding and with a composition suitable for arriving at a desired sponge article. In making the paste it is important to insure that the crystals are quite fine and powder-like and not agglomerated. Crystals which will pass a 30 mesh screen. or finer are satisfactory.

Of the salts known as hydrates of alkali metal salts I find trisodium phosphate more satisfactory than other such salts.

Celluloses derived from wood are generally found satisfactory for the above purposes although other sources of high grade celluloses may be used.

In making sponges according to the invention, one may vary the proportions of the viscose solution to the powdered cellulose over a wide range, dependent upon the properties desired to be incorporated into the final sponge. For example, in certain instances it will be advisable to avoid using powdered or comminuted cellulose whereas in other instances as much as one half or more of the final product sponge will be comprised of powdered cellulose, either mercerized or unmercerized.

The paste is cast into molds or continuously extruded and the cellulose portion of the viscose solution in the paste regenerated by heat to form the cellulose sponge. As a final step the sponge is washed free of processing chemicals and thereafter may be bleached or otherwise purified or even dyed.

In the aforesaid copending applications sponges made according to my invention are set forth including the compositions and structures and advantageous performances of my sponges when compared with currently typical commercial cellulose sponges and cotton tampons.

It is to be noted that, as disclosed in my original application, now US. Patent No. 2,880,726, of which this application is a continuation in part, that a sponge made according to the invention may be so porous that, when dry, each cubic inch thereof contains about 1,400,000 voids, and when wet, its volume expands to some 200% of the original volume, the number of voids being approximately 500,000 to 700,000 per cubic inch.

In accordance with the present invention, sponges having a multiplicity of fine pores to the extent of at least 500,000 pores per cubic inch may be obtained by a basic method wherein a paste is prepared which consists of a viscose solution with or without powdered or comminuted cellulose as described and with powder fine crystals of an alkali metal salt as a pore-forming material. This paste is subjected to an initial elevated temperature in the range of from about 55 C. to about 75 C. for from about A to about one hour depending upon the diameter of the molds and as a result, there is partial regeneration or coagulation of the cellulose of the viscose solution so that the partially regenerated cellulose along with the powder fine crystals of salt, forms a substantially immobile homogeneous gel-like mass. This gelling of this mass during the above initial heat treatment tends to retard the dissolving of the salt in the paste and also retards the migration of the salt crystals in the paste. This mass is next subjected to a temperature in excess of 75 C., for example it may be steamed, so as to effect a more complete regeneration of the cellulose of the viscose solution to form the cellulose sponge. As a final step the sponge is washed free of chemicals, bleached and dried.

The viscose solution portion of the aforesaid paste is prepared by selecting a suitable cellulose for first converting into alkali cellulose, then into cellulose xanthate which is placed in solution by dissolving it with a dilute NaOH solution. For example, when making a tampon sized sponge where good structural stability and tensile strength, volume increase from dry to wet of about 200%, and fluid absorbency of about 27 cubic centimeters per cubic inch of sponge are desired, a cellulose having a cupramrnonium viscosity of about 500 seconds is satisfactory. If a sponge with more structural stability and tensile strength (but with less fluid absorbency and less volume increase than above) is desired, a cellulose having a viscosity in excess of 500 seconds would be selected. Conversely, using a cellulose with a viscosity of less than 500 seconds would result in a sponge with some sacrifice of structural stability and tensile strength but with improved volume increase and fluid absorbency.

Where comminuted cellulose is included in preparing the paste, a sponge with good structural and performance characteristics may be made by using a cellulose having a viscosity of about 18 seconds.

Where mercerized comminuted cellulose is included in preparing the paste, sponges may be made which have a minimum of volume increase from dry to Wet, and a more rapid rate of absorbency of fluid than do sponges containing unmercerized comminuated cellulose, or sponges containing only regenerated cellulose.

While sponges may be made by using sodium sulphate crystals, in a preferred example of my invention I use crystals of trisodium phosphate :as the alkali metal salt in the paste. Sodium sulphate crystals have a low melting point while the phosphate crystals melt at a substantially higher temperature; and, the sulphate is more soluble in water at room temperatures than is the phosphate. Also, when in solution, the sulphate has a pH less than 7 while the phosphate has a pH of more than 7. Thus, phosphate in solution is basic while the sulphate is acidic. Further, crystals of sodium sulphate fine enough to pass a 30 mesh screen are difiicult to make. Sponges made with crystals of sulphate tend to have somewhat larger pores than sponges made from a paste with powder fine crystals of phosphate. Thus, these advantageous properties of the phosphate as a pore forming material lend themselves favorably to the making of sponges containing -a multiplicity of fine pores.

A paste made with sodium sulphate would have a much less workable life, because of the acid nature of the sulphate, compared with a paste made With trisodium phosphate with alkaline properties. Pastes containing sulphate would be made in small lots and molded or extruded promptly. Paste containing phosphate would be made in larger lots and more time allowed for molding or extruding.

Accordingly I prefer to use trisodium phosphate as the alkali metal salt as the pore-forming material of the paste, preferably with crystals fine enough to pass a 30 mesh screen, and advantageously with many crystals much finer than the 30 mesh size.

When formulating the paste it is important that the trisodium phosphate crystal content of the paste be great enough, so that, after taking into account any dissolving of the salt, at room temperatures, in the Water present in the paste, the powder fine crystals will be in substantially intimate contact with each other, and the liquid portion of the paste will fill the very small voids between the salt crystals. Good results have been obtained when the salt content is in excess of of the total paste composition.

The temperature is preferably selected from the range of from about C. to about 75 C. and the time is preferably selected from the range of about A to one hour for the partial regeneration or coagulation of the cellulose of the viscose solution. The temperature and time selected are determined primarily by the diameter of the sponge to be made. For example, when making a tampon size sponge (i.e., about /2 inch in diameter) the mold is about one inch in diameter and the temperature and time selected are 70 C. 75 C. and about 30 minutes. As another example, when making a smaller diameter sponge (i.e., about inch in diameter) the mold is about of an inch in diameter and the temperature and time are C. to C. and about 15 minutes. Larger and smaller diameter sponges would be made with temperatures and times selected according to their diameters. These temperatures and times may also be modified dependent upon the heat transfer through the thickness of the walls of the molds used.

Following this initial or partial regeneration or coagulation of the cellulose of the Viscose solution, the temperature is advantageously increased to above C. to efiect more complete regeneration of the cellulose of the viscose solution, thus in the first example the second temperature applied is about 5 C. higher than the first temperature applied and in the second example the second temperature at least 10 C. higher. At the same time the crystals of the trisodium phosphate are completely dissolved and gases develop due to the complete regeneration of the cellulose. These gases displace the dissolved phosphate forming the multiplicity of fine pores of the sponge. The sponge is then washed free of chemicals and thereafter may be bleached or otherwise purified.

Having now particularly described my invention, what is claimed is:

1. The method of forming a sponge having a multiplicity of fine pores to the extent of at least 500,000 pores per cubic inch, said method comprising preparing a paste including a viscose solution and powder fine crystals of an alkali metal salt having a melting point not greater than about 75 C., subjecting the paste thus formed to two separate heat applications, the first comprising an initial elevated temperature in the range of from about 55 C. to 75 C. to effect partial regeneration of the cellulose of the viscose solution whereby a substantially immobile and homogeneous gel-like mass including the fine crystals of salt is formed and the second heat application comprising thereafter subjecting this gel-like mass to a subsequent temperature at least 10 C. higher than the first applied heat and in excess of 75 C. to accomplish the complete regeneration of the cellulose of the viscose solution to form the sponge.

2. The method of forming a sponge having a multiplicity of fine pores to the extent of at least 500,000 pores per cubic inch, said method comprising preparing a paste including a viscose solution and crystals of an alkali metal phosphate having a melting point not greater than about 75 C. which pass through a 30 mesh screen, subjecting the paste thus formed to two separate heat applications, the first comprising an initial elevated temperature in the range of from about 55 C. to 75 C. to effect partial regeneration of the cellulose of the viscose solution whereby a gel-like mass containing the fine crystals of the alkali metal phosphate is formed and the second heat application comprising thereafter subjecting this gel-like mass to a subsequent higher temperature than the first applied heat by steaming to effect com plete regeneration of the cellulose to form the sponge.

3. The method of forming a sponge having a multiplicity of fine pores, said method as defined in claim 1 wherein said paste includes comminuted cellulose.

4. The method of forming a sponge having a multiplicity of fine pores, said method as defined in claim 1 wherein said alkali metal salt is an alkali metal phosphate.

5. The method defined in claim 4 wherein the alkali metal phosphate is trisodium phosphate.

6. The method as defined in claim 2 wherein the alkali metal phosphate is trisodium phosphate.

7. The method as defined in claim 2 wherein said paste includes comrninuted cellulose.

5 References Cited by the Examiner UNITED STATES PATENTS 2,011,160 8/1935 Plepp 106--164 2,105,380 1/1938 Speijer 106-122 10 2,107,637 2/1938 Carnot 106-122 2,117,392 5/1938 Banigan 106--122 2,133,810 10/1938 Craigue 106--122 2,346,201 4/1944 Vautier 106-122 15 2,464,772 3/1949 Drisch 106-164 2,880,726 4/1959 Stieg. 2,927,034 3/1960 Chih 106-468 ALLAN LIEBERMAN, Acting Primary Examiner.

20 JOSEPH REBOLD, JOHN R. SPECK, MORRIS LIEBMAN, Examiner's. 

1. THE METHOD OF FORMING A SPONGE HAVING A MULTIPLICITY IF FINE PORES TO THE EXTENT OF AT LEAST 500,00 PORES PER CUBIC INCH, SAID METHOD COMPRISING PREPARING A PASTE INCLUDING A VISCOSE SOLUTION AND POWER FINE CRYSTALS OF AN ALKALI METAL SALT HAVING A MELTING POINT NOT GREATER THAN ABOUT 95*C., SUBJECTING THE PASTE THUS FORMED TO TWO SEPARATE HEAT APPLICATIONS, THE FIRST COMPRISING AN INITIAL ELEVATED TEMPERATURE IN THE RANGE OF FROM ABOUT 55*C. TO 75*C. TO EFFECT PARTIAL REGENERATION OF THE CELLULOSE OF THE VISCOSE SOLUTION WHEREBY A SUBSTANTIALLY IMMOBILE AND HOMOGENEOUS GEL-LIKE MASS INCLUDING THE FINE CRYSTALS OF SALT IS FORMED AND THE SECOND HEAT APPLICATION COMPRISING THEREAFTER SUBJECTING THIS GEL-LIKE MASS TO A SUBSEQUENT TEMPERATURE AT LEAST 10*C. HIGHER THAN THE FIRST APPLIED HEAT AND IN EXCESS OF 75*C. TO ACCOMPLISH THE COMPLETE REGENERATION OF THE CELLULOSE OF THE VISCOSE SOLUTION TO FORM THE SPONGE. 